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cgroup: Eliminate cgrp_ancestor_storage in cgroup_root 

The cgrp_ancestor_storage has two drawbacks:
- it's not guaranteed that the member immediately follows struct cgrp in
  cgroup_root (root cgroup's ancestors[0] might thus point to a padding
  and not in cgrp_ancestor_storage proper),
- this idiom raises warnings with -Wflex-array-member-not-at-end.

Instead of relying on the auxiliary member in cgroup_root, define the
0-th level ancestor inside struct cgroup (needed for static allocation
of cgrp_dfl_root), deeper cgroups would allocate flexible
_low_ancestors[].  Unionized alias through ancestors[] will
transparently join the two ranges.

The above change would still leave the flexible array at the end of
struct cgroup inside cgroup_root, so move cgrp also towards the end of
cgroup_root to resolve the -Wflex-array-member-not-at-end.

Link: https://lore.kernel.org/r/5fb74444-2fbb-476e-b1bf-3f3e279d0ced@embeddedor.com/
Reported-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Closes: https://lore.kernel.org/r/b3eb050d-9451-4b60-b06c-ace7dab57497@embeddedor.com/
Cc: David Laight <david.laight.linux@gmail.com>
Acked-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Michal Koutný <mkoutny@suse.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
This commit is contained in:
Michal Koutný 2026-01-07 17:59:41 +01:00 committed by Tejun Heo
parent aa7d3a56a2
commit ef56578274
2 changed files with 15 additions and 12 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

25
include/linux/cgroup-defs.h
View File

@ -626,7 +626,13 @@ struct cgroup {
#endif
/* All ancestors including self */
struct cgroup *ancestors[];
union {
DECLARE_FLEX_ARRAY(struct cgroup *, ancestors);
struct {
struct cgroup *_root_ancestor;
DECLARE_FLEX_ARRAY(struct cgroup *, _low_ancestors);
};
};
};
/*
@ -647,16 +653,6 @@ struct cgroup_root {
struct list_head root_list;
struct rcu_head rcu; /* Must be near the top */
/*
* The root cgroup. The containing cgroup_root will be destroyed on its
* release. cgrp->ancestors[0] will be used overflowing into the
* following field. cgrp_ancestor_storage must immediately follow.
*/
struct cgroup cgrp;
/* must follow cgrp for cgrp->ancestors[0], see above */
struct cgroup *cgrp_ancestor_storage;
/* Number of cgroups in the hierarchy, used only for /proc/cgroups */
atomic_t nr_cgrps;
@ -668,6 +664,13 @@ struct cgroup_root {
/* The name for this hierarchy - may be empty */
char name[MAX_CGROUP_ROOT_NAMELEN];
/*
* The root cgroup. The containing cgroup_root will be destroyed on its
* release. This must be embedded last due to flexible array at the end
* of struct cgroup.
*/
struct cgroup cgrp;
};
/*

2
kernel/cgroup/cgroup.c
View File

@ -5847,7 +5847,7 @@ static struct cgroup *cgroup_create(struct cgroup *parent, const char *name,
int ret;
/* allocate the cgroup and its ID, 0 is reserved for the root */
cgrp = kzalloc(struct_size(cgrp, ancestors, (level + 1)), GFP_KERNEL);
cgrp = kzalloc(struct_size(cgrp, _low_ancestors, level), GFP_KERNEL);
if (!cgrp)
return ERR_PTR(-ENOMEM);